According to one frequent method of implementation, the sensitive structure of a MEMS sensor is made to vibrate so as to observe the effect of the physical quantity to be measured on the vibration amplitude or resonant frequency. In general, the performance of such a sensor depends on the quality factor of the one or more useful vibration modes of the sensitive structure. Single-crystal materials such as quartz or silicon allow micro-resonators with high quality factors to be fabricated.
Their packaging comprises means that allow these MEMSs to be handled during the integration into products without risking degradation thereof and limiting the impact of the external environment on their sensitive structures—in particular to protect them from dust.
To obtain a high quality factor, it is furthermore necessary to place the active structure under a relatively high vacuum. It is therefore encapsulated in a seal-tight enclosure, which may be integrated into the MEMS chip or may be formed by a housing to which the MEMS chip is attached, and within which a low-pressure atmosphere is created. Frequently a pressure lower than 10−2 hPa is necessary for correct operation. The technologies used to attach the MEMS to its housing must therefore be compatible with the generation of a vacuum, meaning that materials liable to degas are to be avoided.
The packaging must also ensure that the relative position of the sensitive structure on its carrier is controllable. It is known to use adhesive bonding or soldering technologies to fasten MEMS to a carrier. An adhesive bond or solder joint is produced on all or part of one side of the structure using what are called flexible silicone-based adhesives or what are called stiff solder joints, which are based on a mixture of tin and gold.
FIG. 1 illustrates a first architecture of an electromechanical device comprising a MEMS chip 500 fastened to a carrier 508. The MEMS chip comprises a stack, produced in a stacking direction z, of a plurality of silicon layers comprising: a single-crystal silicon substrate 501, a cover 504 made of single-crystal silicon, a sensitive layer 502 made of single-crystal silicon and interposed between the cover 504 and the substrate 501, said sensitive layer 502 comprising a deformable sensitive structure 503. The cover 504 and the substrate 501 define a cavity 505 around the active structure 503.
The single-crystal silicon layers 501, 502, 504 are connected pairwise by silicon-oxide layers 506, 507. The MEMS chip 500 is fastened to a carrier 508, here a housing, by means of adhesive or solder bumps 509.
One of the constraints which may affect the performance of MEMS sensors, and that is of particular interest in the present invention, is the need to limit the transmission of slowly varying thermomechanical stresses to the sensitive structure. These thermomechanical stresses are transmitted, to the sensitive structure 503, via the fastening elements 509 of the sensor. These stresses typically originate from:
deformations in the plane perpendicular to the stacking direction z, which deformations are caused by the effect of differential thermal expansion between the material of the MEMS sensor 500 and the material of the carrier 508;
deformations of the carrier 508 out of this plane, for example due to the carrier 508 being fastened to another element;
creep in the material employed to fasten the MEMS sensor to its carrier, i.e. in the fastening bumps, this creep causing a slow variation in the stresses.
Some of these deformations are transmitted to the sensitive structure of the MEMS sensor and cause the output signal of the sensor to drift. It is possible to model and to compensate for some of this drift, in particular the drift due to temperature variations. In contrast, it is not possible to compensate for the drift due to creep. The design of high-performance microsensors requires the architecture of the attachment of the MEMS sensor to be optimized so as to decrease as much as possible the transmission of thermomechanical stresses to the sensitive element, i.e. so as to decouple the static deformation of the carrier and the static deformation of the sensitive structure.
This may in particular be achieved using flexible fastening elements, for example silicone-based adhesives, allowing the carrier to be fastened to the MEMS sensor, and which deform to absorb the deformations of the carrier. However, the adhesive bonding or soldering area must be large enough to ensure the robustness of the joint in the operating environment (vibrations, shocks, thermal cycles). As a result, it is difficult to completely suppress the stresses caused by the difference between the thermal expansion coefficients of the silicon of the MEMS chip and of the adhesive/solder. In addition, the performance of the sensor may be sensitive to geometric defects in the adhesive/solder bump.
It is also known to use soldering technologies with various alloys (for example a mixture of tin and gold) or rigid adhesive bonding technologies applied to areas of the MEMS sensor with various sizes to fasten the MEMS sensor to its carrier. However, since these alloys are much stiffer than a flexible adhesive, they are liable to strongly transmit thermomechanical stresses to the sensitive element.
To mitigate the aforementioned drawbacks, it is known that flexible decoupling structures may be added to form an interface between the fastening elements (or fastening bumps) and the MEMS sensor.
FIG. 2 illustrates a decoupling layer of a second exemplary microsystem according to the prior art. This microsystem is described in the patent application published under the reference EP2447209. It differs from that in FIG. 1 by its cover 604, which is the only element visible in FIG. 2. FIG. 2 schematically shows the cover 604 in cross section. The cover 604 comprises elements F1, F2, F3, F4 via which the MEMS chip is fastened to the carrier 508. The device therefore comprises 4 fastening bumps interposed between the respective fastening elements F1 to F4 and the carrier 508. The fastening elements F1, F2, F3, F4 are joined to those zones of the active layer Z1 to Z4 which are secured to the sensitive layer 502 via flexible decoupling structures that are flexible beams (P1a, P1b, P2a, P2b, P3a, P3b, P4a and P4b) extending in two directions orthogonal to the x,y plane perpendicular to the stacking direction z. The role of the flexible decoupling structures is to filter the static deformations of the carrier in order to minimize the transmission of mechanical stresses from the carrier 508 to the sensitive structure 503.
This solution makes it possible to improve the performance of the static decoupling of thermomechanical stresses between the active structure of the MEMS sensor and its carrier. In contrast, this solution remains limited by the fact that the fastening structure is hyperstatic, i.e. there are more fastening points than is strictly necessary to suppress all the degrees of freedom. Despite the static decoupling provided by the decoupling beams, some of the deformation of the carrier will still be transmitted to the active structure.